[chipotle_coyote]

I'm fairly sure I have seen speed tests between iOS 11 and 12 with actual numbers involved. Someone who has a single phone and upgrades it can't do it, no, but someone who runs, say, a testing lab, or iFixit, or any other place where it's possible to have two different iPhones of the same model running two different versions of iOS can do it pretty easily. Googling "ios 11 vs ios 12 speed" shows a plethora of links from people doing exactly that. I found the same for "ios 11 vs ios 10 speed"; I didn't keep repeating the experiment past that. :)

As to your original point, though -- yes. I'd certainly prefer it if Apple didn't make downgrading so difficult, even if it's just on general principle. I've rarely been tempted to downgrade, but iOS 11 could be pretty clunky on my iPhone 6 compared to its predecessor. (I think 11.4 finally smoothed all that out, but that came out literally a week before iOS 12 was announced!)